Numerical study of high temperature heat exchanger and decomposer for hydrogen production

This dissertation deals with three-dimensional computational modeling of a high temperature heat exchanger and decomposer for hydrogen production based on sulfur-iodine thermochemical water splitting cycle, a candidate cycle in the U.S. Department of Energy Nuclear Hydrogen Initiative. The conceptual design of the shell and plate decomposer is developed by Ceramatec, Inc. The hot helium from a nuclear reactor (T=975°C) is used to heat the SI (sulfuric acid) feed components (H2O, H2SO4 , SO3) to get appropriate conditions for the SI decomposition reaction (T>850°C). The inner wall of the SI decomposition part of the decomposer is coated by a catalyst for chemical decomposition of sulfur trioxide into sulfur dioxide and oxygen. The proposed material of the heat exchanger and decomposer is silicon carbide (SiC).;According to the literature review, there is no detailed information in available publications concerning the use of this type of decomposer in the sulfur-iodine thermochemical water splitting cycle. There is an urgent need for developing models to provide this information for industry. In the present study, the detailed three-dimensional analysis on fluid flow, heat transfer and chemical reaction of the decomposer have been completed. The computational model was validated by comparisons with experimental and calculation results from other researchers.;Several new designs of the decomposer plates have been proposed and evaluated to improve the uniformity of fluid flow distribution in the decomposer. To enhance the thermal efficiency of the decomposer, several alternative geometries of the internal channels such as ribbed ground channels, hexagonal channels, and diamond-shaped channels are proposed and examined. It was found that it is possible to increase the thermal efficiency of the decomposer from 89.5% (baseline design) up to 95.9% (diamond-shaped channel design).;The calculated molar sulfur trioxide decomposition percentage for the baseline design is 64%. The percentage can be increased significantly by reducing reactants mass flow rate and with increasing channel length and operation pressure. The highest decomposition percentage (∼80%) for the alternative designs was obtained in the diamond-shaped channels case.;The sulfur dioxide production (throughput) increases as the total mass flow rate of reacting flow increases, regardless of the fact that the decomposition percentage of sulfuric trioxide decreases as total mass flow rate of reacting flow increases.